Remove redundant "which git" guards

There is always a git executable on PATH due to the git wrapper in
Library/Contributions/cmd. The wrapper will notify the user if there is
not a real git installed.

Library/Contributions/cmd/brew-versions.rb

 require "formula_versions"
 
-raise "Please `brew install git` first" unless which "git"
 raise "Please `brew update` first" unless (HOMEBREW_REPOSITORY/".git").directory?
 raise FormulaUnspecifiedError if ARGV.named.empty?
 

Library/Homebrew/cmd/info.rb

@@ -54,7 +54,7 @@ module Homebrew
   end
 
   def github_fork
-    if which 'git' and (HOMEBREW_REPOSITORY/".git").directory?
+    if (HOMEBREW_REPOSITORY/".git").directory?
       if `git remote -v` =~ %r{origin\s+(https?://|git(?:@|://))github.com[:/](.+)/homebrew}
         $2
       end

Library/Homebrew/cmd/tap.rb

@@ -12,8 +12,6 @@ module Homebrew
   end
 
   def install_tap user, repo
-    raise "brew install git" unless which 'git'
-
     # we special case homebrew so users don't have to shift in a terminal
     repouser = if user == "homebrew" then "Homebrew" else user end
     user = "homebrew" if user == "Homebrew"

Library/Homebrew/cmd/update.rb

@@ -9,7 +9,6 @@ module Homebrew
         Use `brew upgrade <formula>`.
       EOS
     end
-    abort "Please `brew install git' first." unless which "git"
 
     # ensure GIT_CONFIG is unset as we need to operate on .git/config
     ENV.delete('GIT_CONFIG')